According to a related art, for a surface quenching of a cylindrical workpiece, an outer peripheral surface of the workpiece is heated by an induction heating coil (see, e.g., JP2913615B and JP3117008B). The induction heating coil has a fixed coil and a movable coil, and by moving the movable coil in an axial direction of the workpiece, an axial heating range can be adjusted in accordance with the length of the entire workpiece.
According to the induction heating coil described above, the surface of the cylindrical workpiece can be heated along a designated axial length. However, when the workpiece has a cross section that varies along the axial direction, e.g., when a hollow pipe member is partially or entirely depressed inwardly such that a outwardly opened recess portion is provided along a longitudinal axis, this induction heating coil cannot be used.